Conventional battery powered hand tools and other so-called "cordless" devices have self-contained power supplies. Current battery technology does not permit more than a modest stored energy to weight ratio; for their mass, batteries do not deliver very much power. Cordless hand tools, in order to have acceptable operational characteristics, have heavy batteries, and these in turn cause forearm fatigue. The power limitations on cordless devices have limited their use in professional and even homeowner applications. For example, drywallers do not use cordless drills in mounting drywall, as these are too heavy and do not deliver the torque that a 110V AC supplied "corded" drill will.
In order to surmount this problem, workers in the field have developed operator-carried battery packs that are connected to a hand-held tool by an electrical cord. Representative of such configurations are the battery and equipment vest by Rodriguez, U.S. Pat. No. 5,211,321; the battery belt shown in Sing, U.S. Pat. No. 4,748,344; and the battery belt shown in Niecke, U.S. Pat. No. 3,919,615. This last belt also includes an inverter to permit its use with AC powered tools.
While these inventions advantageously shift the weight of the batteries closer to the user, they do not address the power loss caused by the resistance of the electrical cord, which is large when the attached device is drawing a large electrical current. As a result of the voltage drop across the cord, the tool becomes powered at a voltage which is less than its rated voltage, thereby reducing the work which can be performed by the tool when it is most needed. Connections between the power source and the cord frequently create additional losses, compounding the problem. These losses also reduce the time during which the tool can be used before battery recharging is required.
One prior art solution to this problem is to have the power source deliver a voltage to the cord which is significantly higher than the rated voltage of the tool. The drawback of this approach is that under limited load conditions when the tool is drawing less current, the voltage drop across the cord will be less, and the voltage appearing at the tool will be more than the rated voltage. Such overvoltage conditions may cause damage to certain components of the tool, such as the variable speed trigger circuit found in most handheld drills.
A need therefore persists in the industry for methods and apparatus to overcome the voltage drop problem associated with placing the battery supply at a location distant from the tool.